High-solids coating formulations are of great interest owing to their low volatile organic solvents fraction. A core problem, however, is the provision of polyols combining a very low viscosity with the conferment on the cured coating of the necessary mechanical properties, such as scratch resistance or hardness and other resistance properties, such as solvent resistance and chemical resistance.
Coating materials based on polyacrylate-polyols score over polyester-polyols particularly in respect of rapid physical drying, good solvent resistance and chemical resistance and also weathering stability. A problem, however, is their comparatively high viscosity. Polyacrylate-polyols of lower viscosity, such as are needed for high-solids coating compositions, are disadvantageous owing to their low molar weight, since the properties of the coatings produced from them are markedly inferior, particularly with regard to simultaneous scratch resistance, acid resistance and surface hardness, to those of conventional polyacrylate-polyols with a greater solvent content.
DE-A 198 24 118 describes low-solvent binders on a polyester acrylate basis which can be cured with di- and/or polyisocyanates to give quick-drying coatings having good adhesion. Owing to the high polyester fraction, however, the acid resistance of these coatings is inadequate and they are unsuitable for use in automotive topcoat materials.
WO 96/20968 describes a coating composition for cars, vans and lorries that comprises a polyacrylate based on alkyl-substituted cycloaliphatic (meth)acrylate monomers or alkyl-substituted aromatic vinyl monomers, a polyhydroxy-functional oligoester and a polyisocyanate. Since, however, owing to their preparation, the oligoesters have a relatively large number of secondary hydroxyl groups as well as primary ones and since it is necessary to use very large amounts of these esters (<60% by weight, based on the overall formulation) for low-viscosity coating compositions (<3000 mPa.s/23° C.), they cure only very slowly and at relatively high temperatures, and so are unsuitable for temperature-sensitive substrates.
EP-A 0 896 991 describes coating compositions based on polyacrylate/polyester blends with polyester fractions ≦10% by weight and hydroxyl numbers of 40–125 mg KOH/g. Owing to the resultant low crosslinking density, PU coating materials prepared from them lack sufficient solvent and chemical resistance. Moreover, at 3000–5000 mPas/23° C. for a solids content of 70% by weight, the viscosity is too high for the formulation of high-solids PU coating materials.
The present invention was therefore based on the object of providing new binder compositions which have the known advantages of such based on the high molecular weight polyacrylate-polyols but at the same time are of such low viscosity that they are suitable for the preparation of low-solvent coating materials.